~30% of all disease-associated gene lesions generate a premature termination codon (PTC) within an mRNA 2. PTCs reduce gene expression by: 1) terminating translation of a mRNA prior to the synthesis of a full-length polypeptide; 2) eliciting nonsense-mediated mRNA decay (NMD), an mRNA surveillance pathway that degrades PTC-containing mRNAs to prevent their translation. Variability in NMD efficiency among patients that carry PTCs alters the inheritance pattern and the clinical severity of numerous genetic diseases 1, 2. In addition, NMD limits the effectiveness of therapies that suppress translation termination at in-frame PTCs to restore full-length, functional protein 2-5. NMD attenuation therefore represents a potential therapeutic approach to alleviate protein deficiencies caused by PTCs. However, NMD influences endogenous gene expression, multiple cellular pathways, and development 6. In addition, NMD factor insufficiency is associated with intellectual disabilities 7, 8. This indicates that strong NMD inhibition produces harmful effects. We hypothesize that a level of modest NMD inhibition after embryonic development can be imposed without deleterious consequences. Previous NMD-null mice were unsuitable to examine NMD attenuation in vivo because NMD inactivation during embryonic development results in lethality 9. To more rigorously examine the consequences of NMD inhibition in vivo, we generated novel NMD-deficient Tg(dnUPF1) mice that express an inducible dominant-negative UPF1 (dnUPF1) NMD factor. By expressing dnUPF1 in an inducible manner we can: 1) circumvent NMD inhibition during development to allow viability; 2) attenuate NMD at both mild and strong levels in a controlled manner in various tissues. We will use Tg(dnUPF1) mice to examine the consequences of moderating NMD efficiency after embryonic development. This mouse will enable us to examine NMD biology and determine whether NMD is a viable therapeutic target. Specific Aim 1. Examine the effect of in vivo NMD inhibition on mammalian gene expression, morphology, physiology, and behavior. We will induce dnUPF1 expression in a manner that inhibits NMD by various degrees in Tg(dnUPF1) mice. We will then conduct a comprehensive characterization of Tg(dnUPF1) mice to determine: 1) whether NMD inhibition can be maintained over extended periods; 2) whether a level of moderate NMD inhibition can be achieved without deleterious effects; 3) the effects of controlled, long-term moderate NMD inhibition on mammalian gene expression, morphology, physiology, and behavior. Specific Aim 2. Examine the effect of NMD attenuation on PTC suppression in vivo. We have introduced the dnUPF1 transgene into a mouse that carries a genomic nonsense mutation and represents a model for MPS I-H (Hurler syndrome) 10-13. We will examine how dnUPF1 expression affects the abundance of the PTC- containing mRNA as well as the abundance of full-length protein restored by PTC suppression.